A coil spring guide for use in connection with the insertion of catheters into the vessels of a body. Most prior art have circumferentially rounded, outer surfaces that extend more remote from the center axis of the coil spring than the remainder of the guide. These rounded surfaces together with the interstices between the rounded surfaces of adjacent helics, provide a roughened surface that is engageable with the vessels and causes irritation and induces trauma in the vessels as the guide is moved therein, which is undesirable. Another spring coil is formed from a long strip of metal that is rectangular in transverse cross section. However, as a particular matter, when coiling such strips, the coiled strip in transverse cross section forms a concave helical recess extending the length of the strip opening radially outwardly. Grinding or burnishing the outer surface of such a coiled strip to have an outer surface of a substantially round cylindrical configuration could result in the adjacent edges of adjacent helics being sharpened, which when the guide is bent, can scrape against the wall of a vessel. Again, such is undesirable. Further, spring coils of such strips do not have as desirable bending characteristics in going around curves in vessels of the body as spring coils of a circular cylindrical wire as adjacent helics of such strips on one side of the coil have to spread further for a guide of substantially the same strength and outer diameter. In order to minimize or overcome problems such as the above, as well as others, this invention has been made.